The present invention relates to an electronic control system for a carburetor of an internal combustion engine and more particularly to a means for controlling the air-fuel ratio of a mixture to a proper value during the warm-up operation of the engine.
Feedback control systems for controlling the air-fuel ratio are known in the internal combustion engine emission control system art with the three-way catalyst, as disclosed in U.S. Pat. No. 4,132,199. In one of such systems, an oxygen sensor is provided for sensing the oxygen content of the exhaust gases for producing an output voltage as an indication of the air-fuel ratio of the mixture supplied to the engine. An electronic control circuit is provided for actuating an on-off type electromagnetic valve dependent on the output voltage of the oxygen sensor to control the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio. The output voltage of the oxygen sensor varies according to the temperature of the sensor device. More particularly, when the temperature is below 300.degree. C., the output voltage is too low to operate the electronic control circuit for controlling the air-fuel ratio.
In a conventional electronic control system, the duty ratio (the ratio of the duration of valve open period to one on-off cycle of the on-off type electromagnetic valve) is fixed to a predetermined duty ratio during the cold engine operation for providing a lean air-fuel mixture, and, on the other hand, an automatic choke device is provided to correct the lean air-fuel mixture provided by the system to a proper air-fuel ratio dependent on the engine temperature for improving the operability of the cold engine.
The automatic choke device is adapted to close the choke valve by a bimetal dependent on the engine temperature so that the engine may be started in the cold. The choke valve is progressively opened as the engine temperature rises. In the automatic choke device, a slight variation in flow area of the choke valve causes a great variation of the air-fuel ratio. Therefore, it is difficult to control the air-fuel ratio to a desirable value by the automatic choke device.